In the first stage, already underway, we will study experimentally the factors affecting peripheral venous capacity, and will develop a model of its control during exercise and rest as a function of body temperatures and exercise intensity. Such a model will contribute to a more complete understanding of the mechanisms involved in heat syncope and other heat disorders, in which venous pooling plays a prominent etiological role. We measure esophageal temperature (Tes), skin temperatures, and forearm venous volume of subjects exercising on a bicycle ergometer at various combinations of ambient temperature and exercise intensity. Forearm volume is measured with Whitney mercury-in-rubber strain gauges, and venous volume is taken as the difference between the volume measured with veins filled to a standard congesting pressure, and the volume of the forearm elevated with no congesting pressure, and sloping down toward the trunk. Having derived from these measurements a model predicting forearm venous volume in terms of Tes, mean skin temperature, and exercise intensity, we will then investigate how the control parameters of the basic model are affected by a program of exercise training and heat acclimation. We will also investigate the role of the veins in core-to-skin heat transfer by determining the relationship between thermal conductance and skin blood flow at several different levels of venous filling. In addition, we will determine the effect of increased peripheral venous volume on stroke volume, which limits cardiac output under conditions of heat strain. Additional work in this project includes investigating the effects of such factors as age, sex, and season on the control parameters, which will aid prediction of individual tolerance to heat stress.